What is the effect of the side-stream withdrawal rate in a packed distillation column on the temperatures at the top and throughout the column?
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There is a side-line distillation column, with the side-line outlet located above the feed plate. With the feed rate, feed temperature, reflux rate, and steam volume all remaining constant, it was observed that the top temperature of the tower decreased when the amount of product taken from the side stream increased, and increased when that amount was reduced. What is the reason for this?
The section from the side stream extraction layer to the packing layer at the top of the tower is considered as a system. Since the feed rate and steam volume remain constant, an increase in the side stream extraction volume results in a decrease in the gas load from the side stream extraction layer to the top of the tower. With the reflux rate remaining unchanged, the temperature at the top of the tower decreases as well.
With the feed remaining unchanged and more product taken from the side stream, the load on the tower top decreases, while the cold reflux stays the same; as a result, the top temperature drops naturally
The boiling point of the components extracted via the side stream is higher than that of the overhead product; the overhead temperature corresponds to the saturated temperature of the overhead components at that pressure. By extracting more material from the side stream, you are effectively lowering the saturated vapor temperature of the overhead components.
Based on the analysis using the side-line extraction plate, when the flow rate of the rising gas phase remains constant and the heat input is fixed, for this type of plate the flow rate of the descending liquid phase decreases. As a result, the gas-liquid equilibrium temperature on the plate is higher than that at the temperature without side-line extraction, which leads to the situation described by the questioner
You interrupted it halfway and still don’t allow the top to cool down?
Based on the analysis above, the temperature of the entire tower should rise
According to the analysis here. The temperature of the entire tower should rise
By increasing the side-stream yield, more heavy components are extracted from the side stream; as a result, the concentration of heavy components at the top of the tower decreases while the proportion of light components increases, which leads to a drop in temperature. The top temperature represents the composition at a constant pressure.